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994.Rotting Oranges

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This question on LeetCode has been bothering me for a couple of days and afterwards I understood that the typical solution is to use BFS for the cases specified in the description of the question. Therefore, I take a note to let me memorize the solution to this problem next time.

Question

You are given an m x n grid where each cell can have one of three values:

  • 0 representing an empty cell,
  • 1 representing a fresh orange, or
  • 2 representing a rotten orange.
    Every minute, any fresh orange that is 4-directionally adjacent to a rotten orange becomes rotten.

Return the minimum number of minutes that must elapse until no cell has a fresh orange. If this is impossible, return -1.
Example 1:

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Input: grid = [[2,1,1],[1,1,0],[0,1,1]]
Output: 4

Example 2:

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Input: grid = [[2,1,1],[0,1,1],[1,0,1]]
Output: -1
Explanation: The orange in the bottom left corner (row 2, column 0) is never rotten, because rotting only happens 4-directionally.

Example 3:

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Input: grid = [[0,2]]
Output: 0
Explanation: Since there are already no fresh oranges at minute 0, the answer is just 0.

Solution (Good)

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// Time Complexity: O(M*N)
// Space Complexity: O(M*N)
class Solution {
    public int orangesRotting(int[][] grid) {
         // Initial Fresh Oranges
        int freshCount = 0;
        int rows = grid.length, cols = grid[0].length;
        Queue<int[]> queue = new LinkedList<>();

        // This step is to put all the rotten oranges in queue and Count the number of Fresh Oranges
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                if (grid[i][j] == 1) {
                    freshCount++;
                } else if (grid[i][j] == 2) {
                    queue.offer(new int[]{i, j});
                }
            }
        }

        int step = 0;
        // For Movement Usage
        int[][] directions = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
        // BFS starting from initially rotten oranges
        while (!queue.isEmpty() && freshCount > 0) {
            int size = queue.size();

            for (int i = 0; i < size; i++) {
                int[] curr = queue.poll();
                for (int[] dir : directions) {
                    int nextX = curr[0] + dir[0];
                    int nextY = curr[1] + dir[1];
                    if (nextX < 0 || nextX >= rows || nextY < 0 || nextY >= cols || grid[nextX][nextY] != 1) {
                        continue;
                    }
                    queue.offer(new int[]{nextX, nextY});
                    grid[nextX][nextY] = 2;
                    freshCount--;
                }
            }

            step++;
        }

        return freshCount == 0 ? step : -1;
    }
}

Solution (Normal)

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class Solution {
	public int orangesRotting(int[][] grid) {
        int freshCount = 0;
        int rows = grid.length, cols = grid[0].length;
        Queue<int[]> queue = new LinkedList<>();
        List<String> freshList = new ArrayList<>();
        List<String> usedList = new ArrayList<>();

        //record fresh orange
        for(int i=0; i<rows; i++) {
            for(int j=0; j<cols;j++) {
                int status = grid[i][j];
                if(status == 1) {
                    freshList.add(String.format("%d,%d", j, i));
                } else if(status == 2) {
                    queue.add(new int[]{i, j, 1});
                }
            }
        }

        int maxSecond = 0;
        while(!queue.isEmpty() && !freshList.isEmpty()) {
            int[] currRottingOrange = queue.poll();
            int x = currRottingOrange[0];
            int y = currRottingOrange[1];
            int second = currRottingOrange[2];
            maxSecond = Math.max(maxSecond, second);
            if(x+1<cols) {
                String loc = String.format("%d,%d", x+1, y);
                if(freshList.contains(loc)) {
                    freshList.remove(loc);
                    if(!usedList.contains(loc)) {
                        usedList.add(loc);
                        queue.add(new int[]{x+1, y, second+1});
                    }
                }
            }
            if(x-1>=0) {
                String loc = String.format("%d,%d", x-1, y);
                if(freshList.contains(loc)) {
                    freshList.remove(loc);
                    if(!usedList.contains(loc)) {
                        usedList.add(loc);
                        queue.add(new int[]{x-1, y, second+1});
                    }
                }
            }
            if(y+1<rows) {
                String loc = String.format("%d,%d", x, y+1);
                if(freshList.contains(loc)) {
                    freshList.remove(loc);
                    if(!usedList.contains(loc)) {
                        usedList.add(loc);
                        queue.add(new int[]{x, y+1, second+1});
                    }
                }
            }
            if(y-1>=0) {
                String loc = String.format("%d,%d", x, y-1);
                if(freshList.contains(loc)) {
                    freshList.remove(loc);
                    if(!usedList.contains(loc)) {
                        usedList.add(loc);
                        queue.add(new int[]{x, y-1, second+1});
                    }
                }
            }
            second++;
        }

        return freshList.isEmpty() ? maxSecond : -1;
    }
}

Reference:

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